In today's automatic transmissions, a series of clutches or friction elements typically provide a means for application and release of separate members to and from each other during the flow of power through the transmission. These clutches thereby constitute the means by which gears within the transmission are selectively engaged or disengaged typically from either the engine crankshaft or the transmission case. To apply each of these clutches, an electronically controlled hydraulic actuating device, such as a solenoid actuated valve, is often used. There is typically one valve for each clutch. Each of these valves control fluid flow to a respective clutch apply cavity or equivalent arrangement. The flow of fluid into such a clutch apply cavity results in the application or engagement of that clutch. Fluid is typically provided to such a clutch apply cavity from the transmission's fluid pump or other pressurized fluid providing means. This pump provides the required pressurization to allow fluid flow into the clutch apply cavity. Fluid flow is enabled by the opening of the solenoid actuated valve in response to a command or control signal received by the solenoid from an electronic control system.
While the vehicle engine is running, the transmission fluid pump typically maintains fluid within the transmission fluid circuits up to a predetermined level without significant trapped air. When the vehicle engine is turned off, the pump stops pumping and the fluid level within the transmission fluid circuits can drain and drop below this predetermined level. This fluid is often replaced by air. When the engine is subsequently turned back on, the air in the clutch control circuits may not be purged until after the first and/or subsequent applications of that clutch in a shift or shifts of the transmission.
As a result, if the gear shifted into occurs before the air is purged in the respective applying clutch circuit, then additional fluid must replace or compress this air before clutch application can occur. If additional fluid to the respective clutch element is supplied at the lower duty cycle flowrate, then the overall time required to apply the clutch element can be significantly increased. This, in effect, can create a control time lag that can affect shift time and can potentially be perceived by a vehicle operator. Thus, while such shift control systems work well for their intended purpose, there remains a desire for improvement in the relevant art.